Thus, the "mystery" is simply held over, to be attacked by Kant, in whose person eighteenth century thought was to give place to a very contrasted movement. For him, space and time, the general forms of human perception of all events in consciousness, are factors not derived from materials supplied by sensation. They belong to the unifying power of perception in its relation to objects which, again, demands the presence of elements presented by sensation. Accordingly, he is quite clear that, for example, geometrical truth must be classed as a priori; that is, it can not be distilled, as it were, from those sense materials acquired in the course of experience. Thus Kant forces us to class him as a "nativist." So it does not surprise us to observe that he fails to envisage difficulties which were to become capital for physiological psychology at a later time. For instance: How, as a matter of fact, do we construct our completed perception of space? Granted that it be the product of psychical processes, what are they? Granted that it become effective only in the presence of objects, which presuppose sensuous matter, What does this physiological reference gift to our perception? Or, once more, by what subtle alchemy can we explain the obvious fact that we distribute our sensations in space, as it were? How, that is, can we account for localization? Here we quit the philosophical line for a while, observing that its unanswered questions will reappear in an altered perspective.
II
In the realm of physics, prior to the systematic inquiries of the nineteenth century, several more or less sporadic references to the connection between physical and psychical phenomena occur. Such, for example, were the discussions, by Euler and Daniel Bernouilli, of "the law governing the motions of strings";[1] Bernouilli's theory of the mensura sortis,[2] with Laplace's addition of the fortune physique and the fortune morale. These forecast the laws of psycho-physical relationship formulated by E. H. Weber and Fechner. Similarly, the discoveries of Galvani and Volta led to speculations on a supposed parallelism between the known phenomena of electricity and the so-called "discharges" of innervation which, in a way, plumbed the depths of quasi-charlatanism in the developments from Mesmer, and touched the heights of scientific advance in du Bois Reymond's classical work "Untersuchungen über thierische Electricität" (1848), where the mystical and the physical views passed over into physiology for systematic clarification.
Again, Fourier's Law, that "any given regular periodic form of vibration can always be produced by the addition of simple vibrations, having vibrational numbers which are once, twice, thrice, four times,
